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When to Install Protected Left Turn Phasing at a Traffic Signal - Mike on Traffic

When to Install Protected Left Turn Phasing at a Traffic Signal

LTADale Robinson emailed me wondering why some traffic signals with little traffic have protected left turn phasing (solid red arrow) while other signals have protected/permitted or even permitted left turn phasing (just a green circle).  He thinks this is a waste of time and increases vehicle emissions.  He's right!

Standards have evolved over time and often this decision comes down to engineering judgment.  Some traffic engineers don't trust drivers and want all left turn lanes at signals to have protected left turn phasing.  I disagree with that approach and fall in line with NCHRP Report 457, which says on page 42 – "the number of phases should be kept to a minimum because each additional phase in the signal cycle reduces the time available to the other phases."

So traffic engineers should always start with trying to get away with permitted phasing (no arrows).  If that won't work I'm a big fan of the Flashing Yellow Arrow protected/permitted left turn phasing.  This gives motorists a permitted signal most of the time so they can use their judgment, but if traffic is so heavy they can't turn it will eventually give them a green arrow.

There are a few conditions when protected (red/yellow/green arrows) should be used.  NCHRP Report 457 has a good decision tree in Figure 3.5.  Basically, go with protected left turn phasing only if you have one of these conditions:

  • The movement has exceeded the critical number of crashes with the protected/permitted left turn phasing, so protected phasing is required to solve a documented safety issue.
  • There isn't enough sight distance provided – meaning the motorists can't see oncoming traffic for a long enough period to judge if they'll have enough time to make the turn.
  • There are two or more left turn lanes on the approach or the opposing approach.
  • The left turn opposes more than three through lanes.

These situations are pretty rare except at big suburban intersections.  Most left turn lanes at signals should have permitted phasing or flashing yellow arrow protected/permitted phasing.

 

Mike Spack

  • C.R. Hinners says:

    Funny that NHCRP 457 uses “permitted”. I’ve always used “permissive”, as does the MUTCD and NHCRP 493. After all, a protected left is “permitted”, as is a permissive left. 😉

  • PawelMF says:

    Someone should send this article to NHDOT. They have a policy on protected left-turns statewide.

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